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KABUL, Afghanistan — An Afghan soldier turned his gun on American troops at a checkpoint in the country's east, killing two Americans and at least two fellow members of Afghanistan's army in a shooting that marked both the continuance of a disturbing trend of insider attacks and the 2,000th U.S. troop death in the long-running war, officials said Sunday.

The string of insider attacks is one of the greatest threats to NATO's mission in the country, endangering a partnership key to training up Afghan security forces and withdrawing international troops.

Saturday's shooting took place at an Afghan army checkpoint just outside a joint U.S.-Afghan base in Wardak province, said Shahidullah Shahid, a provincial government spokesman.

"Initial reports indicate that a misunderstanding happened between Afghan army soldiers and American soldiers," Shahid said. He said investigators had been sent to the site to try to figure out what happened.

An Afghan official speaking on condition of anonymity said three Afghan soldiers were killed in the clash. It was not clear if the assailant was among the dead.

The attack happened about 5 p.m. in Sayd Abad district, Afghan Defense Ministry spokesman Gen. Zahir Azimi said in an emailed statement. He did not provide further details, saying he would wait for a report from investigators.

NATO forces announced the assault early Sunday morning, saying only that it was "suspected insider attack" and that a NATO service member and civilian contractor were killed.

One U.S. official confirmed that the service member killed was American, while another confirmed that the civilian was also American. The officials spoke on condition of anonymity because the nationality of the dead had not yet been formally announced.

Afghan soldiers and policemen — or militants in their uniforms — have gunned down more than 50 foreign troops so far this year, eroding the trust between coalition forces and their Afghan partners. An equal number of Afghan policemen and soldiers also died in these attacks, giving them reason as well to be suspicious of possible infiltrators within their ranks.

The attacks are taking a toll on the partnership between international and Afghan forces, prompting the U.S. military to restrict operations with small-sized Afghan units earlier this month.

The close contact — with coalition forces working side by side with Afghan troops as advisers, mentors and trainers — is a key part of the U.S. strategy for preparing the Afghans to take the lead in security operations as the U.S. and other nations prepare to pull out their last combat troops at the end of 2014, just 27 months away.

The number of American military dead reflects an Associated Press count of those members of the armed services killed inside Afghanistan since the U.S.-led invasion on Oct. 7, 2001.